We sometimes use \indexlibrarymember when indexing "global" library entities (in std namespace really). Elsewhere, we use \indexlibraryglobal. Why? What should the convention be?

For example, with the addition of P2819R2, Joel E. pointed out that tuple_size and tuple_element are actually "global", yet the convention used for similar entities (e.g. tanh) was to index them as if they were members of complex. See #6693 (comment)

Note that elsewhere we've also used indexes like the following:
\indexlibrary{\idxcode{formatter}!specializations!\idxcode{nullptr_t}}%

With \indexlibrarymember{x}{y}, we get y under x in the index which might be helpful, but we also get x under y which might not make sense, and we don't get x by itself. In the cases in question, neither x nor y is a member, so using ,\indexlibrarymember to index them is confusing. Note that we're also inconsistent as to whether we write \indexlibrarymember{x}{y} vs. \indexlibrarymember{y}{x}, so we need to be careful when fixing this issue.

Originally posted by @burblebee in #6693 (comment)